Police on Monday said they had found 87 bomb detonators at a Colombo bus station, a day after a string of attacks on churches and hotels that killed nearly 300 people.

A statement said police found the detonators at the Bastian Mawatha Private bus stand, 12 of them scattered on the ground and another 75 in a garbage dump nearby.

The Sri Lankan government believes a local extremist group called National Thowheeth Jama'ath (NTJ) was behind deadly suicide bomb attacks that killed nearly 300 people, government spokesman Rajitha Senaratne said on Monday.

Senaratne, who is also a cabinet minister, added that the government was investigating whether the group had "international support."

Documents seen by AFP show Sri Lanka's police chief issued a warning on April 11, saying that a "foreign intelligence agency" had reported NTJ was planning attacks on churches and the Indian high commission.

Authorities lifted a curfew in the Sri Lanka on Monday, a day after 290 people were killed and about 500 wounded by a string of bombings that tore through churches and luxury hotels on Easter Sunday.

There was still no claim of responsibility for the attacks on two churches and four hotels in and around Colombo, the capital of predominantly Buddhist Sri Lanka, and a third church on the country's northeast coast.

A government source said President Maithripala Sirisena, who was abroad when the attacks happened, had called a meeting of the National Security Council early on Monday. Prime Minister Ranil Wickremesinghe would attend the meeting, the source said.

There were fears the attacks could spark a renewal of communal violence, with police reporting late on Sunday there had been a petrol bomb attack on a mosque in the northwest and arson attacks on two shops owned by Muslims in the west.

Sri Lanka had been at war for decades with Tamil separatists but extremist violence had been on the wane since the civil war ended 10 years ago.

The South Asian nation of about 22 million people has Christian, Muslim and Hindu populations of between about eight and 12 percent.

The island-wide curfew imposed by the government was lifted early on Monday, although there was uncharacteristically thin traffic in the normally bustling capital.

Soldiers armed with automatic weapons stood guard outside major hotels and the World Trade Centre in the business district, where the four hotels were targeted on Easter Sunday, according to a Reuters witness.

Scores of people who were stranded overnight at the main airport began making their way home as restrictions were lifted.

The government also blocked access to social media and messaging sites, including Facebook and WhatsApp, making information hard to come by.

Wickremesinghe acknowledged on Sunday that the government had some prior information about possible attacks on churches involving a little-known extremist group, but said ministers had not been told.

Sri Lankans accounted for the bulk of the 290 people killed and 500 wounded, although government officials said 32 foreigners were also killed. These included British, US, Turkish, Indian, Chinese, Danish, Dutch and Portuguese nations.

A British mother and son eating breakfast at the luxury Shangri-La hotel were among those killed, Britain's The Telegraph newspaper reported.

One Australian survivor, identified only as Sam, told Australia's 3AW radio the hotel was a scene of "absolute carnage".

He said he and a travel partner were also having breakfast at the Shangri-La when two blasts went off. He said he had seen two men wearing backpacks seconds before the blasts.

"There were people screaming and dead bodies all around," he said. "Kids crying, kids on the ground, I don't know if they were dead or not, just crazy."

There were similar scenes of carnage at two churches in or near Colombo, and a third church in the northeast town of Batticaloa, where worshippers had gathered for Easter Sunday services. Pictures from the scene showed bodies on the ground and blood-spattered pews and statues.

Dozens were killed in one of the blasts at the Gothic-style St. Sebastian church in Katuwapitiya, north of Colombo. Police said they suspected that blast was a suicide attack.

Three police officers were also killed when security forces raided a house in Colombo several hours after the attacks. Police reported an explosion at the house.

Sri Lankan security personnel keep watch outside the church premises following a blast at the St. Anthony's Shrine. AFP